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Parr IL. § vii] DETERMINATION OF ROCKS. 183 
thickness of each slice must be mainly regulated by the nature of 
the rock, the rule being to make the slice as thin as can conveniently 
be cut, so as to save labour in grinding down afterwards. Perhaps 
the thickness of a shilling may be taken as a fair average. The 
operator, however, may still further reduce this thickness by cutting 
and polishing a face of the specimen, cementing that on glass in 
the way to be immediately described, and then cutting as close as 
possible to the cemented surface. The thin slice thus left on the 
elass can then be ground down with comparative ease. 7 
Excellent rock-sections, however, may be prepared without any 
machine, provided the operator possesses ordinary neatness of hand 
and patience. He must procure as thin chips as possible. Should 
_ the rocks be accessible to him in the field, he should select the 
freshest portions of them, and by a dexterous use of the hammer 
break off from a sharp edge a number of thin splinters or chips, out 
of which he can choose one or more for rock-slices. These chips 
may be about an inch square. It is well to take several of them, 
as the first specimen may chance to be spoiled in the preparation. 
The geologist ought also always to carry off a piece of the same 
block from which his chip is taken, that he may have a specimen 
of the rock for future reference and comparison. Lvery such hand- 
specimen, as well as the chips belonging to it, ought to be wrapped 
_ up in paper on the spot where it is obtained, and with it should be 
placed a label containing the name of the locality and any notes 
that may be thought necessary. It can hardly be too frequently re- 
iterated that all such field-notes ought as far as possible to be written 
down on the ground where the actual facts are before the eye for 
examination. 
Having obtained his thin slices, either by having them slit with a 
machine or by detaching with a hammer as thin splinters as possible, 
the operator may proceed to the preparation of them for the mi- 
eroscope. For this purpose the following simple apparatus is all that 
is absolutely needful, though if a grinding-machine be added it will 
save time and labour. 
List of Apparatus required in the Preparation of Thin Slices. of Rocks and 
Minerals for Microscopical Examination. 
. A cast-iron plate 1 inch thick and 9 inches square. 
Two pieces of plate-glass, 9 inches square. 
A Water of Ayr stone, 6 inches long by 23 inches broad. 
Coarse emery (1 1b. or so at a time). 
. Fine or flour emery (ditto). 
. Putty powder (1 0z.). 
. Canada balsam. (There is an excellent kind prepared by 
Rimmington, Bradford, specially for microscopic preparations, and sold in 
shilling bottles.) 
8. A small forceps, and a common sewing-needle with its head fixed 
in a short wooden handle. 
SIS OTR OD bo 
